1. Technical Field
This invention relates to a solid-state imaging device with substantially improved dynamic range.
2. Related Art
Solid-state imaging devices (also referred to as image devices or imagers) have broad applications in many areas including commercial, consumer, industrial, medical, defense and scientific fields. Solid-state imaging devices convert a received image from an object into a signal indicative of the received image. Solid-state imaging devices are fabricated from semiconductor materials (such as silicon or gallium arsenide) and include photosensitive imaging arrays (photosensors) of light detecting picture elements, or pixels, (also known as photodetectors) interconnected to generate analog signals representative of the received image. Examples of solid-state imaging devices include charge coupled devices (CCD), photodiode arrays, charge injection devices (CID), hybrid focal plane arrays and complementary metal oxide semiconductor (CMOS) imaging devices.
Photosensors of the solid-state imaging devices are typically formed in an array structure, with rows and columns of photodetectors (such as photodiodes, photoconductors, photocapacitors or photogates) which generate photo-charges proportional to the radiation (such as light) reflected from an object and received by the photosensor. The period of exposure of a photosensor by incident radiation is referred to generally as the integration period. An exposure shutter may control exposure of the photosensor to incident photons. The exposure shutter may be, for example, electrically, mechanically or electro-magnetically operated. The photo-charges are created by photons striking the surface of the solid-state (i.e. semiconductor) material of the photodetectors within the photosensor. As photons strike a photodetector, free charge carriers (i.e., electron-hole pairs) are generated in an amount proportional to the incident photon radiation. The signals from each photodetector may be utilized, for example, to display a corresponding image on a monitor or to provide information about the optical image.
Each photodetector includes a detecting area (also known as the photosensitive area or the detector area) and photodetector circuitry within a common integrated circuit die. The photodetectors receive a portion of the reflected light received at the solid-state imaging device, and collect photo-charges corresponding to the incident radiation intensity falling upon the photodetectors' detecting area of the die. The photo-charges collected by each photodetector are converted to an output analog signal (analog charge signal) or a potential representative of the level of energy reflected from a respective portion of the object. The analog signal (or potential) is then converted to a digital voltage value and processed to create an image.
Solid-state imaging devices are commonly utilized in digital camera devices for both still picture and video applications. In these types of applications, video or still picture quality is related to dynamic range. As result, it is desirable to obtain a digital video or still picture of scenes with a large dynamic range. Despite the differences in CCD, CID and CMOS imager technologies, these technologies typically have the common problem of limited dynamic range. The dynamic range is defined by the maximum number of photons that a photodetector may collect during a period of photon exposure (also referred to as an integration period) without saturating (i.e., exceeding the capacity of) the photodetector, and the minimum number of photons that a photodetector may collect during the integration period that may be detected over the noise floor. More specifically, the dynamic range is defined as the ratio of the effective maximum detectable signal level (often referred to as “saturation”) with respect to the root-mean-square (RMS) noise level of the photosensor.
Solid-state imaging devices that generate photo-charges due to incident photons, such as CCD, CID and CMOS imaging devices, have a dynamic range that is limited by the amount of charge that is collected and held in a given photodetector. As an example, if the saturation of a particular photodetector in a solid-state imaging device is 20,000 electrons, and the incident light on that photodetector is so bright that it creates more electrons than may be held in the photodetector (i.e., greater than 20,000 electrons), the excess charge is lost because the excess electrons do not contribute to the signal corresponding to that photodetector. In general, the dynamic range problem is more problematic when the photodetector is an active pixel sensor (APS) cell (i.e., the cell incorporates an active component such as a transistor within the pixel) as compared to a passive pixel sensor (PPS) cell, due to the active components in the APS cells which limit the area available for the detector area, and due to the low voltage supply and clocks utilized in APS cells.
In addition to lost excess charge, excess carriers (i.e., hole-electron pairs) that exceed the amount of charge capable of being stored in the photodetector may cause an undesired blooming phenomenon. Blooming occurs when the excess carriers that exceed the saturation level are locally or partially generated, and those excess carriers flow to other photodetectors. In order to avoid blooming, the exposure time of the image may be decreased. However, when the exposure time is lowered, the photodetectors corresponding to the darker portions of the image collect an insufficient amount of charge to provide meaningful information (i.e., the collected charge may be indistinguishable from noise).
Past attempts at solving the limited dynamic range problem have utilized a non-integrating active pixel sensor cell with a non-linear load device, to obtain a logarithmic response. This approach, however, has a number of disadvantages. First, the noise in a non-integrating cell is much higher than the noise in a conventional integrating cell. Also, the exact non-linear transfer function of this type of device is carefully calibrated to avoid variations from cell to cell and compensate for temperature changes. This increases the complexity and inaccuracy of the device.
Another attempt at solving the problem of limited dynamic range was to increase the resolution of the analog-to-digital converters of the solid-state imaging device. However, the higher the precision of an analog-to-digital converters, the slower it operates. This results in a reduced frame rate for a given number of photodetectors and a given number of analog-to-digital converters. Further, as the resolution of an analog-to-digital converter is increased, the least significant bits begin to fill with system noise (including noise from the conversion circuitry itself) rather than meaningful photodetector magnitude information.
Finally, another attempt at solving the limited dynamic range problem utilized a higher supply voltage for the photodetector circuitry so as to increase the charge capacity of each photodetector. A problem with this approach is that the maximum supply voltage that may be utilized is reduced as semiconductor fabrication processes continue to shrink chip sizes for cost and power advantages.
Thus, a need exists for an improved solid-state imaging device with a substantially increased dynamic range that overcomes the problems and limitations associated with past systems.